DixoN AND Ball — Channels of Transport in Seedlings. 1 9 1 



The bundles after entering the haustorium consist of 8-10 tracheae and a few 

 wood-parenchyma cells, forming their xylem ; their phloem is composed of large 

 phloem-parenchyma cells, which separate the two or three groups of sieve-tubes. 

 Between this phloem and the xylem there are two or three layers of cambiform 

 cells. 



Epidermal cells of haustorium, diameter ... 001-0-06mm. 



Sieve-tubes, diameter ... ... ... 001-002mm. 



Sieve-tubes, length ... ... ... 010-015 mm. 



Tracheae, diameter ... ... ... OOl-OOo mm. 



Vicia faha. 



With a view to throwing some further light on this subject, the anatomy of 

 the cotyledons of the broad bean, Vicia faha, was also studied. In the plant, as 

 is well known, the cotyledons do not emerge from the seed-coat, but function 

 solely as storage organs; and, being normally below the ground, they do not 

 transpire. 



Sections of a seed which had been soaked in water for twenty-four hours were 

 first examined. The Iralk of the cotyledons is composed of large parenchjonatous 

 cells, which are closely packed with starch and protein granules. Between these 

 cells there is a considerable development of intercellular spaces. Where the 

 neighbouring cells come in contact with one another there are numerous large 

 pits in the intervening walls, which possibly facilitate diffusion from cell to cell. 

 The cotyledons are traversed by vascular bundles, which even at this stage show 

 fully developed vessels with spiral thickenings; but there is no trace of sieve- 

 tubes, the remainder of the bundle being composed of elongated parenchymatous 

 cells filled with protoplasm and containing large nuclei. This early development 

 of wood in the cotyledons may be contrasted with the conditions obtaining in the 

 radicle and plumule, in which the presence of xylem cannot be recognized until 

 some time after germination. 



At a later stage, when the main root has reached a length of about 12 cm., 

 the wood in the cotyledons shows a considerable increase in amount, and well- 

 developed pitted vessels are present. The exact time at which the sieve-tubes 

 are differentiated in the cotyledons is difficult to determine. At this stage they 

 are certainly present in the petioles, and at a later stage can also be seen in the 

 laminae of the cotyledons. The mature sieve-tubes can be easily recognized by 

 the presence of the peculiar ellipsoidal slime-masses in the vacuole of the cell. 

 These bodies were described by Strasburger (13) and Beccarini (1) in members 

 of the Ijeguminosae, and are particularly well-developed in Vicia. They appear 

 to be quite free in the sieve-tube, and in most cases they lie close to one of the 

 sieve-plates. 



The anatomy of these cotyledons, in so far as it has a bearing on the problem 

 of translocation, may be briefly considered. The storage tissue through which 

 the soluble organic materials must pass by diffusion is constructed so as to 

 facilitate this function as far as possible. The vascular bundles, on the other 

 hand, probably provide the conduit for transport to the growing points. The 

 early formation and later development of the xylem in the cotyledons, where 

 transpiration is negligible, is significant. It may, of course, be considered that 

 the xylem is a purely vestigial structure, but its extremely marked development 

 hardly seems in accordance with this view. If, however, it is functional, it 

 would seem that it must be of use in translocation. We know that organic 

 substances must at times be able to pass with comparative freedom from cell to 



